For diabetics, determining the blood glucose concentrations and corresponding medication are an essential part of the course of the day. In this case, the blood glucose concentration has to be determined rapidly and simply a number of times a day (typically two to seven times) in order to be able to take corresponding medical measures, if appropriate. In many cases, medication is effected by means of automatic systems, in particular so-called insulin pumps.
In order not to restrict the diabetic's day any more than necessary, correspondingly mobile devices are often used, which should be simple to transport and handle, so that the blood glucose concentration can be measured without any problems, for example at the workplace or else during leisure time. Various mobile devices which function in part according to different measurement methods and using different diagnosis methods are commercially available at the present time. A first measurement method is based on an electrochemical measurement method. For example, a blood sample, taken from the body tissue from the patient for example by perforating a skin layer by means of a lancet, is applied to an electrode coated with enzymes and mediators. Corresponding test strips for electrochemical measurement methods of this type are described in U.S. Pat. No. 5,286,362, for example, the disclosure of which is hereby incorporated herein by reference. Other known measurement methods use optical measurement methods based for example on the fact that the substance (analyte) to be detected may react with specific detection reagents, a change in the color of the reaction mixture occurring. Systems for detecting such color reactions and thus for detecting the corresponding analytes are known from CA 2,050,677, for example, the disclosure of which is hereby incorporated herein by reference.
The detection methods described are therefore based predominantly on the fact that a patient firstly takes a corresponding sample of the body fluid to be examined (this being for example a blood sample or a urine sample) and then examines it correspondingly by means of the test device. However, this method comprises various disadvantages. Firstly, this method is extremely complicated and presupposes a plurality of handling steps. Thus, by way of example, a lancet has to be provided and tensioned, a skin layer subsequently has to be perforated by means of said lancet, a drop of blood thus produced then has to be applied to a test strip, and said test strip subsequently has to be evaluated by means of a corresponding device. For many patients, in particular older people and children, these handling steps can often be carried out only with difficulty since the patients are restricted for example in terms of their motor ability and their vision capability. Furthermore, these method steps can be carried out discretely only in a few cases, such that for example protection of the patient's privacy during a measurement at the workplace is only inadequately ensured. Moreover, incorrect operation of the measurement method can easily lead to false measured values, with in some instances fatal consequences of an incorrect medication based on the measurement results.
The prior art therefore discloses systems which can be implanted into a body tissue and which supply measured values continuously. See, for example, U.S. Pat. No. 6,892,085 and U.S. Pat. No. 5,591,139, the disclosures of which are hereby incorporated herein by reference in their entireties.
Overall, however, the implantable sensors known from the prior art are extremely complicated with regard to their construction and their production. If it is assumed that said sensors are disposable sensors that can only be used for a short time (typically approximately one week), then it becomes clear that the complicated manufacturing methods used for making the sensors: known from the prior art do not satisfy such requirements made of disposable articles. See, for example, the lithographic methods disclosed by U.S. Pat. No. 5,591,139 and U.S. Pat. No. 6,892,085 which are referenced supra. However, such methods cannot be reconciled with the production of cost-effective disposable articles.
Moreover, lithographic methods, in particular the etching of metal layers that is associated with these methods, are not always as reliable as necessary for producing medical-technological products. In particular, it can happen that individual electrodes are still connected to one another by “bridges”, such that the functionality of the sensors can easily be impaired or even completely prevented on account of production problems. A further disadvantage of the sensors known from the prior art, such as, for example, the sensors known from U.S. Pat. No. 6,892,085 B2 and U.S. Pat. No. 5,591,139, furthermore arises in the use of a hollow needle or capillary. In these cases, the sensors are introduced into a capillary which transports the body fluid to be examined towards the sensor. What is disadvantageous about this, however, is that the capillary makes it more difficult for the analyte solution to have unimpeded access to the electrodes. In particular, this can also give rise to local concentration corruptions which have the effect that measurement results do not correspond to the actual concentration conditions in the body fluid. In this case, complex diffusion processes and flow processes in the capillaries also play a part and contribute to the corruption.
Other prior art sensors are provided for in vivo measurement based on an electrochemical principle, having two electrodes on a carrier substrate. See, for example, US 2004/0111017, the disclosure of which is hereby incorporated by reference in its entirety. In such in vivo sensors, a working electrode coated with a detector layer for the analyte to, be detected is applied directly to the carrier substrate and covered by a covering layer. A common reference electrode and counter electrode can be applied on the opposite side of the covering layer to the working electrode and overlaps the working electrode but is isolated from the latter by the covering layer (which is necessarily to be configured in electrically insulating fashion). The analyte passes via diffusion mechanisms from the edges of the sensor to the working electrode. As an alternative, the covering layer itself can also be made analyte-permeable.
Such a sensor arrangement has various disadvantages in practice, however. One exemplary disadvantage is the fact that the covering layer must simultaneously perform two different functions which are compatible only with difficulty in terms of material technology. Thus, the covering layer must on the one hand have sufficient electrically insulating properties in order to insulate the working electrode and the counter electrode from one another. The covering layer must nevertheless enable the analyte that is to be detected to penetrate at least from the edge in order to pass to the working electrode, in order to be able to be detected electrochemically there. This diffusion must able to take place to a sufficient extent in order to be able to provide sufficient responsive electrical currents for a measurement of the analyte concentration (signal response). The simultaneous permeability for diffusion and sufficient insulation capability make stringent requirements of the material properties, however. One solution for solving this problem is a structural configuration of the sensor in which diffusion channels enabling the analyte to penetrate are provided in the layer construction. However, this structure is technically so complicated that the production advantages which can be afforded by a layer construction are virtually completely given away again.
It is an object of the invention, therefore, to provide a sensor for determining a concentration of at least one analyte in a medium, which sensor can be produced simply and cost-effectively by means of a reliable production method and if possible avoids the disadvantages of the sensors and methods known from the prior art. In particular, the sensor is intended to be implantable and to ensure sufficient signal swings.